Catalytic supports based on magnesium compounds, particularly magnesium chloride (MgCl2), are the most effective for the production of Ziegler-Natta (ZN) catalysts for olefin polymerization. Perhaps one of the main advantages to use an inorganic carrier for a ZN catalyst is the control of the morphology, which enables the production of polymer with predictable shape, bulk density and particle size distribution due to the replica phenomenon. Given the importance of the support nature and its crystallization, it is important to explore alternatives routes for support preparation.
Since the early 1970s, the patent literature and scientific articles have described routes to prepare spherical support from magnesium halides. One of the most important and widely used routes to obtain spherical and porous particles is by oil emulsion. This method is described, for example, in the U.S. Pat. Nos. 4,469,648, 4,399,054, 5,578,541, and 6,861,385. Such documents describe a catalyst obtained from a support based on the formation of an emulsion of a fused product between MgCl2 and alcohol at high temperature (120° C.) and pressure (9.8 bar) followed by subsequent precipitation of this emulsion in a non-solvent medium at low temperature. Furthermore, U.S. Pat. No. 6,323,152 teaches that a long contact period (longer than 10 hours) is required for a complete melting between the mixture components in oil (MgCl2 and alcohol).
In another approach, spherical particles are obtained through a Spray-Drying technique, as mentioned in U.S. Pat. No. 6,982,237, EP0123767, U.S. Pat. Nos. 4,376,062, and 4,311,817. In this case, the solution is fed to the equipment by pumping and passing it through an atomizer at high temperature. The solvent evaporates in a chamber to form spherical dried particles. The product is separated in the cyclone from the gas and it is collected in a vessel followed by a solvent recovering step from the upper part of the cyclone. However, in this process it is necessary to control several parameters, for example; feeding rate, gas flow to spray, temperature of gas flow to spray, flow rate of carrier gas in a chamber, and temperature of carrier gas. Additionally, this process involves evaporation of the solvent for subsequent precipitation of the solid, which contributes to the particle formation. Thus, this technique is highly dependent on experimental conditions. For example, rapid removal of alcohol can lead to formation of fragile hollow particles, which are not suitable for the production of ZN catalysts due to the poor mechanical strength of the particles.
On the other hand, U.S. Pat. No. 4,421,674 describes catalyst supports which are obtained through a similar Spray-Drying technique. It describes the application of a solution of MgCl2 with alcohol, which is heated from 40 to 100° C. This solution is then dispersed through a spray nozzle and the solvent evaporates at high temperature, approximately 180° C. In this case, a large amount of solvent is carried into the Spray -Dryer, reducing the holding capacity of the solvent (vapor) in the gas stream and which may result in high content of the solvent in the final product. It is worth noting that limited information was given concerning the support morphology in U.S. Pat. No. 4,421,674, and the catalyst obtained was evaluated only for ethylene polymerization carried out at low pressure.
Another route to spherical support production is through a Spray-Cooling technique, as mentioned in U.S. Pat. No. 4,829,034. A solution, suspension or melted product is atomized into a spray of fine droplets of spherical shape inside a spray cooling chamber. In this case the droplets meet the inert gas stream at low temperature, which solidifies the droplets. A mixture of magnesium compound, alcohol, and internal electron donor in molten state is then pumped to a nozzle and sprayed droplets meet the cold inert gas or fluid, which flows from bottom to top of the spray cooling chamber, to form spherical particles. European Patent EP0700936 describes a process for producing a solid catalyst component for olefin gas phase polymerization from a mixture of a magnesium compound with an alcohol which is sprayed in a molten state into a spray column. Simultaneously, the inside of the spray column is cooled down to a temperature at which a solid component (B) is obtained without any substantial vaporization of the alcohol in the mixture (A), to obtain the solid (B), followed by partial removal of the alcohol from the solid (B) between 20° C. and 60° C. under reduced pressure, to obtain a solid component (C).
In both cases a molten mixture is used and sprayed into a spray column containing a gas or fluid at low temperature to obtain a solid component catalyst. In this process, like the Spray-Drying process, is necessary to control many variables such as: feeding rate, gas flow to spray, flow rate of carrier gas in a chamber, temperature of the carrier gas, heated pipe for transferring the solution above 100° C., homogeneity of gas throughout the system, temperature control to maintain the gas supply, and the precipitation chamber. All these factors may lead to a more complex and expensive process.
In yet another approach, patent application WO2014095523A1 describes the use of a spray cooling technique as an alternative process to solidify a fused Mg(OR1)2 and alcohol adduct in a cold liquid, in the absence of an inert liquid dispersant. The melted mixture of Mg(OR1)2 and alcohol could be sprayed through a device in a low temperature environment to cause the solidification of particles. Thus, it suggests that the support preparation process undergoes processing under high temperature and pressure conditions.